The OKT9 monoclonal antibody reacts with human CD71, a 170-180kDa type II transmembrane protein. CD71, the transferrin receptor, exists as a homodimer on the cell surface and is essential for cellular growth. CD71 is expressed by immature proliferating cells and at low levels on resting mature lymphocytes. Antigen or mitogen stimulation of T and B cells upregulates the expression of CD71.